These teachings relate generally to fuel cell feed systems, and particularly to a vapor feed direct oxidation fuel cell.
A fuel cell induces an electrochemical reaction between fuel such as methanol or hydrogen and oxygen to convert chemical energy directly into electric energy, and therefore high power generation efficiency is obtained; the noise is very low, and the size can be reduced because there are few mechanical moving parts. Such a fuel cell is relatively easy in installation and management, and is hence used in distributed power supply systems, and power supplies for communication equipment, etc.
Portable electronic devices such as cell phones and laptop computers are extremely reduced in size and weight and enhanced in performance. Secondary batteries such as high performance nickel metal hydride batteries and lithium ion batteries are conventionally used as the power source.
For these conventional secondary batteries, it is difficult to extend the operating time and reduce the size and weight further. Accordingly, fuel cells are candidate power sources for portable electronic devices.
Direct oxidation fuel cells that use liquid fuel through direct oxidation at the anode are most promising to replace batteries because of the potentially higher energy density. When methanol is used as the fuel, it is called direct methanol fuel cell (DMFC), which is most widely investigated. In a DMFC, methanol and air are supplied to the fuel cell anode and cathode, respectively, generating electricity, plus heat and water. A conventional DMFC uses dilute methanol solution of not higher than 3 M, and typically not higher than 1 M. Due to the presence of a large percentage of water in 1 M methanol aqueous solution (about 97% wt.), such a DMFC system gives up too much of the achievable energy density.
Fuel cells developed at MTI Micro Fuel Cells Inc. use neat methanol (i.e., 100% methanol) to feed the fuel cell anode without adding any water to either the fuel or the ambient air. The liquid methanol is vaporized once it gets into the fuel cell, and the resulting methanol vapor transports to the anode catalyst layer to be subsequently oxidized.
Fuel feed into a controllable vapor feed DMFC presents some unique challenges. A fuel feed system for such a fuel cell should have, in the smallest possible package, the ability to substantially constantly feed fuel at typically very low feed rates against a variable vapor back pressure from the fuel cell side, mainly due to the enormous volume expansion when the liquid methanol is vaporized to vapor. With variable back pressure from the fuel cell, coupled with the requirement for controllable feed rates of fuel according to the varied power output requirements by the load, a successful fuel feed system should be responsive accurately to the fuel flow requirements.